The actions of hormones and drugs which inhibit adenylate cyclase are mediated by specific receptors and an inhibitory GTP-binding regulatory protein (GI). Guanine nucleotides decrease the affinity of receptors for their ligands in many systems. This is thought to indicate that the receptor is coupled to GI. Although other labs have demonstrated this phenomenon for the muscarinic receptor in membranes, we have solubilized the receptor-GI complex from rat heart membranes and have demonstrated guanine nucleotide sensitivity of receptor affinity in the solubilized system. Further, we have separated the receptor-GI complex from uncoupled receptor and are continuing to characterize these proteins in an attempt to elucidate the molecular mechanism of adenylate cyclase inhibition.